with all that I've seen, I would be amazed if it wasn't the BT's router that is acting up.

Them routers can start acting up with 1 client on the network. You should not rely on that device to do your DNS (on top of other stuff it is already doing which are on top of actually routing your traffic) but have a dedicated in house server. What comes to root hints not working at several clients, something was clearly wrong and now you can never know what caused that to fail. Switching to the router as "dns proxy" just isn't the best solution (how remote-hackable that thing is?)

for cable issues, the switch will be able to tell you bad drops with error counters. If you have bad traffic instead (flood for example), you would want to put a monitoring system in place.
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